FreeNAS - Bi-Directional Rsync Over SSH

Go to /root on 1st server.
ssh root@server1
cd /root

FreeNAS OS drive is mounted read-only, so mount it RW.
mount -o rw /

Generate an RSA key & leave the passphrase blank.  You can use another supported algorithm if you wish.
ssh-keygen -t rsa

Display the public key.
more .ssh/id_rsa.pub

Copy / paste the key to a text editor and remove any line breaks.  The key should appear on 1 line.  Leave the spaces intact (e.g. the space after "ssh-rsa").

Copy / paste the key to /root/.ssh/authorized_keys on the 2nd server.

Repeat the above process for the 2nd server.  Create the key on the 2nd server, and copy/paste it to the 1st.

Test your new SSH keys by SSH-ing into the 2nd server from the 1st and vice-versa.  The first time you'll get prompted to trust the key.  Accept the prompt to add the host to known_hosts.  Disconnect and SSH again.  It should connect immediately without prompting for a password.
ssh root@server1

The keys have been swapped successfully, so you can now setup your rsync jobs.